Method of metallizing silica-containing gel and solid state light modulator incorporating the metallized gel

ABSTRACT

A solid state light modulator structure useful in a video display system includes a deformable silica containing gel layer on an array of charge storage elements, and an adherent, highly light reflective metal (e.g., Ag) electrode layer formed directly on the surface of the gel layer by sputtering in a non-reactive atmosphere.

This is a continuation of application Ser. No. 084,262, filed Aug. 11,1987, now abandoned.

CROSS-REFERENCE TO RELATED APPLICATION

Co-pending U.S. patent application, Ser. No. 084,260, filed concurrentlyherewith, claims a method for metallizing a silica-containing gel, inwhich the gel surface is treated with an oxygen-containing plasma priorto metallization, and a solid state modulator incorporating themetallized gel.

BACKGROUND OF THE INVENTION

This invention relates to a solid state light modulator structure usefulin an apparatus for generating an image from a video input signal, andmore particularly relates to such a structure including a deformablemetallized elastomer layer, and also relates to a method for metallizingthe layer.

U.S. Pat. No. 4,626,920 describes a video display system of the typeemploying solid state light modulator structures including a deformable,metallized elastomer layer. In this light modulator structure, thedeformable layer, for example, a metallized silica-containing gel layer,is disposed over an array of electrodes on the surface of a solid statecharge storage device, such as a charge coupled semiconductor device.The metal layer on the surface of the gel serves both as an electrodeand as a light reflecting layer.

In operation, electric fields associated with the charge pattern of astored video frame cause electrostatic attractions between the electrodearray and the surface electrode, resulting in deformation of the gellayer in a pattern corresponding to the charge pattern. This pattern isanalagous to a phase diffraction grating. The information contained inthis pattern is then "read" by reflecting light from the deformedelectrode into an optical subsystem such as a Schlieren type of opticalsystem, which then translates the information into a viewable image on ascreen.

A critical step in the formation of these light modulator structures isthe formation of the light reflective electrode layer on the gelsurface. In order for the device to operate successfully, such layermust be electrically conductive, flexible and adherent to the gel layer,and is preferably highly specularly reflective. Unfortunately, sincegels are semisolids, having structures which may be characterized ashaving a liquid phase distributed in an open polymer network, theyprovide poor surfaces for adhesion.

In the referenced U.S. patent, an electrode layer with the desiredcharacteristics is provided by first providing a thin pellicle layer ofnitrocellulose on the gel surface, to provide a surface for adhesion,and to isolate the electrode layer from gel components which couldattack and degrade it. Next, a thin intermediate layer of gold isevaporated onto the pellicle layer, followed by evaporation of a thinsilver layer onto the gold layer. The gold layer enables the depositionof the silver layer with sufficient uniformity to result in a highlyreflective layer.

While it would be desirable from the standpoint of manufacturingefficiency to eliminate the intermediate pellicle and gold layers, ithas been found that the deposition of silver directly onto the gelsurface by evaporation results in an extremely low specular reflectance,that is, less than one percent. G. C. Martin et al., J. Appl. Phys. 53(1), 797 (1985).

Accordingly, it is a principal object of the invention to provide asolid state light modulator structure of the type described hereinhaving an adherent, highly reflective metal layer on the surface of thegel layer.

It is another object of the invention to provide a method for forming anadherent, highly reflective metal layer directly onto asilica-containing gel surface without the need for forming intermediateor transitional layers.

SUMMARY OF THE INVENTION

In accordance with the invention, there is provided a solid state lightmodulator structure comprising:

(a) a solid state charge storage device comprising an array of chargestorage elements formed in a semiconductor substrate, each elementassociated with at least one display electrode on the surface of thesubstrate,

(b) a deformable elastomer layer disposed on the surface of the chargestorage device, the layer covering the array of display electrodes, and

(c) a flexible, adherent and light reflective conductive layer disposedover the elastomer layer,

characterized in that the elastomer layer is a silica-containing gel,the conductive layer is a metal layer, and the metal layer is bondeddirectly to the gel layer.

Such a solid state light modulator structure is further characterized inaccordance with the invention in that the metal layer is selected fromthe group consisting of silver, aluminum and indium, and is preferablysilver, having a specular reflectance of visible light of at least 90percent.

In accordance with another aspect of the invention, there is provided amethod for forming an adherent, light reflective metal layer directlyonto the surface of a silica-containing gel, the method comprisingsputtering the metal onto the gel surface in a nonreactive atmospheresuch as an inert gas.

Such sputtered metal atoms arrive at the gel surface with much greaterkinetic energy (one or more orders of magnitude greater) than if suchatoms were evaporated. This high energy is thought to be responsible forthe formation of a chemical bond between the metal atoms and siliconatoms at or near the gel-metal layer interface. The sputtered metallayer is thus adherent to the gel surface and highly reflective. Wherethe metal is silver, layers having specular reflectances greater than 90percent are obtainable.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE is a cross-section view of one embodiment of a solidstate light modulator structure in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the FIGURE, a solid state light modulator 10 comprises a chargestorage device including semiconductor substrate 12, such as silicon,including an array of charge storage elements (not shown) formed in thesubstrate, each charge storage element associated with at least onedisplay electrode 14 on the substrate surface. Such a charge storagedevice may, for example, comprise a charge coupled device. A detaileddescription of the structure and operation of these devices isunnecessary to an understanding of this invention, and may be found, forexample, in U.S. Pat. No. 3,882,271.

Disposed over the top of the charge storage element array is adeformable elastomer layer 16, herein a silica-containing gel. Anadherent, light reflective metal electrode layer 18 is disposed on thetop surface of elastomer lay 16, completing the light modulator.

In operation, a charge array stored in the light modulator structurerepresenting for example, a video frame, in conjunction with a potentialapplied between the array of display electrodes and the upper lightreflective electrode layer, results in a variation of potential acrossthe gel layer 16, and electrostatic attraction forces between theelectrode array 14 and the light reflective electrode 18, causingdeformation of gel layer 16 and reflective layer 18. Such deformationresults in a rippled pattern on the surface of the gel and the lightreflective layer 18, which pattern can then be "read" by reflectinglight from the surface into a Schlieren optical system, which translatesthe pattern into a visual display image. Such an optical system and itsoperation are described in more detail in the above-referenced U.S. Pat.Nos. 4,626,920 and 3,882,271.

As may be appreciated, the successful and efficient operation of themodulator structure is dependent upon the electrode layer 18 havingsufficient flexibility and sufficient adherence to the gel surface sothat it faithfully reproduces the deformations in the gel layer 16. Inaddition, the electrode layer 18 is preferably highly specularlyreflective, so that the largest possible amount of light incident on thesurface is reflected back into the optical system for display of thestored image.

However, attempts to simply evaporate a metal layer such as silver ontothe surface of a silica-containing gel such as a polydimethyl siloxane(PDMS), results in poor adhesion to the surface and a very low specularreflectance (less than one percent). In accordance with the teachings ofthe invention, it has been discovered that forming the metal layer bysputtering in an inert gas such as argon results in a layer which ishighly specularly reflective (greater than 90 percent in the case ofsilver) and which adheres well to the gel surface. Such a layer isbelieved to result from a modification of the gel surface by the plasmapresent in the sputtering environment, by changing the crosslinking inthe gel surface, and promoting a chemical reaction between the sputteredmetal and the gel surface. The crosslinking is believed to make the gelsurface more rigid, while the chemical reaction is believed to improvethe bonding between the gel layer and the sputtered metal layer.

The sputtering may be carried out over a wide range of operatingconditions. For example, the gas pressure may range from about 50 toabout 200 micrometers, and the current may range from about 20 to about500 milliamps. As is known, the sputtering deposition rate generallyincreases with increasing current, and the current increases withincreasing gas pressure. As is also known, a high deposition rate mayresult in undesirable heating of the substrate. In such cases,deposition may be carried out in stages, with rest periods to allowcooling of the substrate to occur. In the alternative, provision may bemade for actively cooling of the substrate in order to allow forcontinuous deposition.

The gel layer should have a modulus of elasticity within a range toallow the required amount of deformation, which is determined by devicegeometry as well as the wavelength of the light to be modulated. By wayof example, red light having a wavelength of from about 550 to 570nanometers may be modulated in a structure having a gel layer about 10microns in thickness and having a modulus of elasticity between about104 and 106 dynes per square centimeter. When a field of about 50 voltsis impressed across the gel layer between the electrode array on thesurface of the semiconductor and the flexible metal electrode on thesurface of the gel, an electrostatic attraction results in deformationof the gel by an amount of up to about 0.2 micrometers.

Particularly suitable for use as the gel layer in these structures arepolydimethyl siloxanes (PDMS), which may be readily synthesized bycuring a mixture of A and B components, where A is dimethylvinylterminated polydimethyl siloxane and B is trimethyl terminated siloxanewith partially hydrogen-substituted methyl side groups. These componentsare commercially available, for example, from the Dow Chemical Companyunder the tradename Dow Sylgard 527. Gels having various moduli ofelasticity may be synthesized simply by varying the weight ratio of A toB. For example, varying the weight ratio of A:B from 1:1 to 1:2 resultsin a modulus of elasticity variation of approximately an order ofmagnitude. As is known, the modulus of elasticity may also be varied bychanging the molecular weight of the A component, for example by washingor fractionation, and by changing the functionality of the B component,defined as the number of H side groups, for example, by the sametechniques.

EXPERIMENTAL Sample Preparation

Dow Sylgard 527 compounds were used for the synthesis of PDMS gels withA:B weight ratios of 1:1, 1:1.25, 1:1.5, 1:2, and 1:5. The averagemolecular weight distribution of both A and B components was broad andcentered around 20,000 grams/mole. The components were mixed in theliquid state and formed into thin layers by spinning onto 17×17millimeter monoscope substrates. Monoscopes were used instead of siliconsemiconductor chips in order to enable study of the viscoelasticproperties of the gels. The layers were gelled by curing them at atemperature of about 100° C. for a time of about one hour. The thicknessof the cured layers was about 10 micrometers.

Silver layers characterized as thick (about 1,000 angstroms) and thin(about 20 angstroms) were then sputtered onto the PDMS gel layers usingsputtering conditions as follows: argon gas at a pressure of 160micrometers, current at 200 milliamps, source-target distance at about11/8 inch. The thick samples were produced by sputtering in pulses of 15seconds duration, followed by 5 minutes of rest, while the thin sampleswere produced by a single pulse of less than 5 seconds duration. Thesilver thickness in the thin samples was determined by assuming a linearrate of silver deposition and extrapolating gravimetric measurements forthe thick samples.

A silver standard for X-ray photoelectron spectroscopy (XPS) wasprepared by sputtering a thick silver layer onto a quartz substrateusing the same sputtering conditions described above. A gel standard wasprepared having an A:B ratio of 1:1.5 and having a surface free of anydeposited metal. A gold standard was also formed by evaporating a thingold layer on to a gel layer having an A:B ratio of 1:1.

Surface Reflectivity

The thick silver layers obtained by sputtering are highly specularreflective (greater than 90 percent) and have a pale yellow color.

XPS

XPS studies were conducted using a magnesium K.sub.α source at 300 wattswith an analyzer including a triple detector having a resolution of 1.05electron volts at 20 electron volts of pass energy, measured at thesilver 3d_(5/2) peak. The gold standard was assigned a peak value of83.8 electron volts. The difference between this peak and the actualgold peak of 92.4 electron volts was assumed to be due to samplecharging, and this value was used for assigning positions to the peaksin the silver samples.

Survey scans of both the thick and thin silver samples were conducted atlow (from about 5 to 605 electron volts) and high (from about 600 to1200 electron volts) binding energy, using 0.2 electron volt steps at500 milliseconds per channel and a detector pass energy of 50 electronvolts.

High resolution scans were then recorded in the vicinity of the silicon2p, silicon 2s, carbon 1s, oxygen 1s, and silver 3d peaks, using 0.05electron volt steps at 500 milliseconds per channel and a detector passenergy of 20 electron volts. The peak positions for the silver standard,the thin silver samples and the gel standard obtained from these scansare listed in Table I. All peak positions except the Auger peakpositions were measured from the high resolution scans. The Auger peakpositions were determined from the survey scans.

                                      TABLE I                                     __________________________________________________________________________          Silver                   Gel                                            Element                                                                             Standard                                                                           1:1 1:1.25                                                                            1:1.5                                                                             1:2 1:5 Standard                                       __________________________________________________________________________    Si 2p --   102.10                                                                            102.05                                                                            102.10                                                                            102.10                                                                            102.00                                                                            102.20                                         O 1s  --   532.05                                                                            532.00                                                                            532.00                                                                            532.10                                                                            532.00                                                                            532.30                                         C 1s  --   284.45                                                                            284.35                                                                            284.45                                                                            284.45                                                                            284.35                                                                            284.50                                         Ag 3d 5/2                                                                           368.10                                                                             367.85                                                                            367.85                                                                            367.90                                                                            367.90                                                                            367.85                                                                            --                                             Ag α                                                                          527.6                                                                              528.4                                                                             528.5                                                                             528.4                                                                             528.3                                                                             528.2                                                                             --                                             __________________________________________________________________________

Auger Parameter of Sputtered Silver

Table I shows the silicon 2p, oxygen 1s, carbon 1s, and silver 3d_(5/2)peak positions corrected for charging, and the Auger parameter forsilver (α) for the thin samples, the silver standard and the gelstandard. The variations in the peak positions are within experimentalerror except for the silver Auger parameter α which has shiftedconsiderably (about 0.8 electron volts) in the thin samples, indicatinga change in the chemical state of the silver sputtered on to the gelsurfaces. The thick samples did not show such a shift in α, indicatingthat a chemical bond is formed between the sputtered silver and the gelsurface at or near the gel-silver layer interface.

Areas under the peaks on the survey scans were measured, and the ratiosof the areas under the silver peaks to the areas under the peaks forsilicon, oxygen and carbon were computed. These ratios are listed inTable II for the thick and thin silver samples.

                  TABLE II                                                        ______________________________________                                        A to B              Ag/                           Ag/                         Ratio O      Ag     O    C    Ag   Ag/C  Si  Ag   Si                          ______________________________________                                        Silver Thickness (A) 20                                                       1:1   19.5   80.5   4.1  18.0 82.0 4.6   4.4 95.6 21.6                        1:1.25                                                                              16.5   83.5   5.1  15.0 85.0 5.7   4.3 95.7 22.4                        1:1.5 15.6   84.4   5.4  16.2 83.8 5.2   4.3 95.7 22.1                        1:2   17.8   82.2   4.6  15.9 84.1 5.3   4.7 95.3 20.2                        1:5   14.3   85.7   6.0  15.7 84.3 5.4   3.5 96.5 28.0                        ______________________________________                                        Silver Thickness (A) 1000                                                     1:1   16.2   84.8   5.2  11.3 88.7 7.9   4.8 95.2 19.8                        1:1.25                                                                              11.8   88.2   7.5  7.4  92.6 12.6  2.5 97.5 39.0                        1:1.5 10.7   89.3   8.3  8.0  92.0 11.5  1.2 98.8 81.0                        1:2   13.2   86.8   6.6  8.6  91.4 10.6  2.9 97.1 33.8                        1:5   15.9   84.1   5.3  11.0 89.0 8.1   4.2 95.8 22.6                        ______________________________________                                    

Etching and Etch Rate of Gels

The survey scans for both the thick and thin samples showed silicon 2pand other peaks associated with the gel structure, indicating thatetching of the gel occurred during sputtering.

Table II shows the ratios of the silver peak areas to the silicon,oxygen, and carbon peak areas for the thick and thin samples withvarying ratios of A:B. The data for the thick samples indicates that theamount of etching depends upon the degree of gel crosslinking. The peakarea ratios of the thick samples reaches a maximum (silver/silicon=81.0)for the A:B ratio of 1:1.5, apparently indicating that the etch rate isslower in the 1:1.5 samples and faster in the 1:1 and 1:5 samples, withthe etch rates of these latter samples being approximately the same.

While there have been shown and described what are at present consideredto be the preferred embodiments of the invention, it will be apparent tothose skilled in the art that various changes and modifications may bemade therein without departing from the scope of the invention asdefined by the appended claims.

What is claimed is:
 1. A solid state light modulator structurecomprising:(a) a solid state charge storage device comprising an arrayof charge storage elements formed in a semiconductor substrate, eachelement associated with at least one display electrode on the surface ofthe substrate, (b) a deformable elastomer layer disposed on the solidstate device, the layer covering the electrode array, and (c) aflexible, adherent and light reflective conductive layer disposed overthe elastomer layer, characterized in that the elastomer layer ispolydimethyl siloxane gel, the conductive layer consists of a singlesputtered metal layer, and the metal layer is bonded directly to the gellayer.
 2. The solid state light modulator of claim 1 in which the metalin the conductive metal layer is selected from the group consisting ofsilver, aluminum and indium.
 3. The solid state light modulator of claim2 in which the metal is silver.
 4. The solid state light modulator ofclaim 3 in which the specular reflectance of the silver layer is atleast 90 percent.